Play along with me while we consider the ultimate barometers of economic wellbeing that are fertility rates and births across the developed world, China, and the RoW (rest of the world). How fertility rates have freefallen to predominantly negative rates. How the developed nations fertility rates turned negative first, then China, then most the RoW (& soon nearly the entire world).
A quick perusal of the chart of global fertility rates, by income groups below, shows a sharp and sustained drop in global fertility since 1970. It is well understood how (and why) China did this...less well understood how (and why) the developed world got there first and has sustained it for decades. My supposition is that declining fertility rates worldwide are primarily due to central bank set interest rate policies which have consistently encouraged debt and asset inflation well ahead of real income inflation. These rising costs of living have particularly punished those with little or no offsetting assets...chief among these, young adults among the childbearing population. The decades long squeeze has slowed marriages to a crawl and sent births tumbling. Given family formation and childbirth are a choice now with widely available contraception...the choice is increasingly, no.
Data below is from UN World Population Prospects 2019...future fertility and birth estimates are a more realistic average between UN median and low estimates.